23
used to evaluate the drag forc
e op
posed by
tillage implements as well as an
i
ndex for soil
resistance to root development (e.g. C.I. < 2
MPa is often found acceptable).
As a consequence of the above
-
mentioned
effects, soil moisture and balance are strongly
affected by type of tillage. This comes from:
°
soil drying facilitated by soil loosening;
°
enhanced water intake, especially when
tillage is combined with residue mulching and
improved soil structure stability;
°
reducing direct evaporation from deeper
untilled soil layers (mulching effect from both
soil mulching and residue
-
mulching);
°
controlling weeds that disperse water taken
up from deep layers.
The effects on runoff and soil erosion after a
rainfall event mainly depend on slope, soil
infiltration rate and
s
oil cover (vegetation or
residue mulch) and from any ditches and their
direction on the slope. The infiltration rate can be
high in rough soil left after til
l
age, provided the
soil has a stable clod structure and crust does not
form. More than runoff, soi
l
erosion is reduced
by vegetation and residue mulch (e.g. down to
about 1-10%); large amounts of residues (e.g.
from maize) are more efficient than small ones
(e.g. soybean) [2].
Chemical effects of tillage are not immed
i
ate; the
related processes, howeve
r
, start to develop at
once and evolve over time. They are interpreted
in terms of soil aeration (modes and intensity) and
mechanical displacement of materials through the
soil pr
o
file, the latter effect being magnified by
soil
inversion or mixing operatio
n
s (
Fig. 13
). The
most spectacular effect comes from
e
nhanced
organic matter (o.m.) decay through the stages of
humus formation and final mi
n
eralization
(according to a first approxim
a
tion law o.m.
decreases asymptotically) [28]. It is essentially
from the
aeration promoted by tillage that this
decrease in soil o.m. content (often to about one
half to one third that of virgin soil) occurs over a
number of decades; this usually implies
degradation of soil structure to a certain degree
and
i
ncreased need for
m
ore accurate and
intensive agronomic practices to maintain the
yield at satisfactory levels (an old Italian farming
adage is that "once you start ploughing your soil,
you have to plough it for ever"). The final steady
state level depends on general pedocl
i
matic
conditions and agronomic pra
c
tices; and almost
all arable, still fertile land in the world is subject
to this rule. If these general conditions are
unfavourable, ho wever, as may happen in some
semi-arid areas, drought can follow the ploughing
[29].
A
ll nutrients are gradually released; of
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